Radioactive materials are often detected and identified by measuring the energy spectrum of radiation (e.g. gamma rays, x-rays, neutrons, etc.) that they emit. The spectrum of energy of the emitted radiation is specific to that particular material and thus acts as a “fingerprint” to identify the material.
The ability to detect gamma rays and/or neutrons is a vital tool for many areas of research. For example, gamma-ray/neutron detectors allow scientists to study celestial phenomena and diagnose medical diseases. Additionally, these detectors are important tools for homeland security, helping the nation confront new security challenges. The nuclear non-proliferation mission requires detectors capable of identifying diversion of or smuggling of nuclear materials. Government agencies need detectors for scenarios in which a terrorist might use radioactive materials to fashion a destructive device targeted against civilians, structures, or national events.
Thallium Bromide (TlBr) has emerged as a promising wide-bandgap semiconductor crystal suitable for room temperature radiation detection. However, radiation detectors comprising TlBr, despite presenting high resolution and accuracy, invariably degrade after operation times ranging from mere hours to months. This degradation is typically attributed to the polarization of the crystal due to the accumulation of oppositely charged Tl+ and Br− ions in the TlBr crystal under an applied bias. Ionic movement in the TlBr crystal is mediated mostly by Tl and Br vacancies, which occur naturally in the intrinsic or pristine material in quantities that vary according to the temperature. These intrinsic positively and negatively charged vacancies are generated in equal proportions and are called Schottky pairs. Vacancies can also be induced extrinsically via doping with a charged impurity. In order to maintain charge neutrality, additional oppositely charged vacancies will be generated in response to these impurities. When an electric field is applied to TlBr, not only photo-excited electrons, but also ions start to move towards the electric contacts. The greater the quantity of vacancies, the larger the ionic current will be. Ultimately, the resulting imbalance in the distribution of charged ions produces an internal electric field that opposes the direction of the applied bias, thereby decreasing the collection of charge carriers and thus degrading the spectroscopic performance, e.g. the signal energy resolution, of TlBr crystals.
Several conventional techniques for reducing the degradation (e.g. reducing the polarization) and increasing the stability of TlBr crystals have been attempted. One approach is to cool the TlBr crystal down to about −20° C. or lower, which serves to decrease the ion mobilities. In addition to increasing the operation costs of the device due to the need of sustaining this low temperature, TlBr still experiences the temporal degradation observed at room temperature, although over a longer time frame.
Other attempted approaches to improve performance and stable operation of a TlBr crystal include: applying Tl metal contacts to the TlBr crystal: periodically reversing the bias field (e.g. roughly every 24 hours); ultrapurification of TlBr crystal samples; and mechanical and/or chemical surface treatment of the TlBr crystal. However, these conventional techniques are also associated with several disadvantages. For instance, the use of Tl metal contacts may be unfavorable, as Tl metal is highly toxic, reactive with elements in the environment, and may be readily absorbed through the skin. Moreover, periodic switching of the bias fields adds to the complexity and cost of the circuitry, and may not be compatible with single carrier charge sensing techniques. Moreover still, ultrapurification of TlBr crystal samples may both time expensive and time-consuming.
Thus, while the above conventional techniques may increase the operational lifetime of TlBr crystals employed for radiation detection, none have achieved a long lasting solution to the degradation observed in TlBr radiation detectors. New approaches are therefore needed make possible the large scale application of TlBr as an economically viable radiation detector material.
Accordingly, it would be beneficial to provide systems and methods for stabilizing TlBr crystals without suffering from the drawbacks associated with the conventional techniques described above.